1. Field of the Invention
The present invention relates to semiconductor devices. More particularly, the present invention relates to a diffused resistor formed in a PN junction isolation type semiconductor device.
2. Related Arts
The most common type of resistive element that has heretofore been integrated in PN junction isolation type semiconductor devices is a P-type diffused resistor, which is formed by diffusing impurities, e.g., B (boron). That is, an N.sup.- -type epitaxial layer is formed on a P-type semiconductor substrate, and a P.sup.+ -type isolation region is formed in the N.sup.- -type epitaxial layer, and further a P.sup.+ -type region is formed in the N.sup.- -type epitaxial layer, thereby forming a diffused resistor. Usually, the PN junction between the P.sup.+ -type resistor region and the N.sup.- -type epitaxial layer is reverse-biased at all times. More specifically, the potential of the N.sup.- -type epitaxial layer is set to the highest potential of the circuit, generally the power-supply voltage.
However, in a diffused resistor that is used in a circuit portion where a voltage higher than the power-supply voltage may be applied as an input voltage, the N.sup.- -type epitaxial layer is connected nowhere and placed in a floating potential state. The reason for this is that, if the potential of the N.sup.- -type epitaxial layer is set to the power-supply voltage as described above, when the voltage input to the resistor exceeds the power-supply voltage, the above-described PN junction is forward-biased, and a current flowing at this time causes breakage of a contact with wiring, e.g., aluminum wiring, or fusing of the wiring.
It is known that, when a diffused resistor is used under high-temperature environmental conditions where the temperature may exceed 100.degree. C., current may leak from the epitaxial layer (N-type) to the substrate (P-type). In such a case, if the potential of the epitaxial layer has been set to the power-supply voltage, the leakage current is derived from the power supply. Accordingly, the leakage current has no effect on the current flowing through the resistor layer, and gives rise to no problem in practical use.
However, in a case where the epitaxial layer is placed in a floating potential state, that is, in a circuit configuration where a voltage higher than the power-supply voltage may be applied to the resistor, the current flowing through the resistor layer is drawn to form the above-described leakage current. Moreover, the leakage current may turn on a parasitic PNP transistor which is formed from the substrate, the epitaxial layer and the resistor layer. If the parasitic PNP transistor is activated, the amount of current drawn from the resistor layer is increased by the amplification factor of the transistor, causing the circuit characteristics to be disordered when the resistor layer is used as an input resistor for protecting a circuit element.
The leakage current may be reduced by providing an N.sup.+ -embedded region between the P-type substrate and the N-type epitaxial layer to thereby lower the amplification factor of the parasitic PNP transistor. With this technique, however, the amplification factor of the parasitic PNP transistor cannot satisfactorily be lowered.